1. Field of the Invention
This invention relates to sorting of waste materials for recycling, and in particular to a method and apparatus for distinguishing paper objects from plastic objects in order to separate the paper objects from the plastic objects.
In particular, the invention relates to apparatus for distinguishing paper from plastic objects by using a conveyor, a laser and detector placed on opposite sides of the conveyor so that light is transmitted through the objects to be distinguished, and a thresholding circuit or software for distinguishing paper objects from plastics based on whether the transmitted light is above or below the threshold.
While it is well-known to analyze different types of objects based on the properties of light transmitted through the objects, it was not previously appreciated that paper and plastic articles could be distinguished in this manner. The invention is made possible by the discovery of the Inventor that paper and plastic objects have non-overlapping transmissivities with respect to coherent light when positioned a sufficient distance away from the detector, allowing paper and plastic objects to be distinguished through the use of a single light source and detector, and analysis by thresholding. The discovery was made by the Inventor, age 14, as part of a science fair project.
2. Description of Related Art
(a) Introduction
The prior or related art includes numerous systems and methods for distinguishing glass from plastic and different types of plastics from each other using light transmission, but none for distinguishing paper from plastic, and none are currently in use in recycling plants or sorting facilities.
In order to distinguish plastic or glass objects from each other, complex spectral analyses and/or the use of electromagnetic wavelengths outside the visible spectrum are required. The resulting apparatus and methods are too complex or expensive to implement in a typical recycling plant or waste sorting facility. In contrast, the present invention is designed to be implemented with a minimum of equipment, and used in a typical sorting or recycling facility of the type that currently uses manual labor to separate paper from plastic articles.
Even though sorting by analysis of transmission or reflection spectra are known for the purpose of separating different plastics from each other, for separating glass from plastic, or for sorting such objects as fruit, the only currently available methods of separating paper articles from plastic articles are to separate the articles by hand, by using jets of air, or by elutriation. Generally, sorting and recycling of paper and plastic articles is a low margin activity, and the complex setups required of prior optical sorting arrangements have made them unattractive even for investigation with respect to sorting of paper and plastic articles.
The societal benefits of recycling paper, plastic, and other types of solid waste are of course obvious and, in most municipalities, special trucks regularly pick up recyclable waste and cart it to transfer stations or directly to recycling facilitates, which lessens the load at incinerators and landfills. In order to recycle the received materials, however, the materials must initially be separated into plastic, glass, metal, and paper. Glass and metal objects can easily and economically be separated from the waste stream by air classification or magnets, but the weight and other properties or plastic and paper articles make them more difficult to distinguish for purposes of separation.
In plants or facilities where air classification or elutriation is used, a significant amount of paper is missed and must be removed either through float/sink tanks or hydroclones. As a result, most organizations that collect waste for recycling still sort paper and plastic by hand.
The present invention uses a very simple arrangement involving an inexpensive monochromatic source laser and a detector that measures the intensity of light passing through the objects. The arrangement is sufficiently simple and effective that it is believed that a number of waste facilities would have adopted the method and apparatus if known, rather than rely on hand sorting or relatively inefficient air classification or elutriation. However, it has only become apparent that such a sorting method would work based on the results of the Inventor's investigations, which are described below in connection with FIG. 3. Basically, the invention is made possible by the discovery that paper and plastic have non-overlapping transmissivity spectra when the objects are placed between a laser source and a detector, at a minimum distance from the detector that can readily be determined. The Inventor is not aware of any reason why the spectra should not overlap, but the results summarized in FIG. 3 show that they do not.
(b) Discussion of Specific References
Many of the references directed to sorting of objects by analysis of transmitted light are directed to separation of materials such as different types of plastics that have more subtle differences in physical properties than is the case with paper and plastic, and that require complex measuring and analysis methods not suitable or economical for use in separating paper from plastic. For example, U.S. Pat. No. 5,141,110 discloses a system that measures the intensity of light passing through different types of plastics such as PET and PVC or vinyl using polarization filters in order to measure birefringence or polarizing properties of the materials. This method requires polychromatic light sources, polarizing filters, and relatively sensitive detectors, and is only suitable for separating very specific types of plastics having different crystalline structures.
U.S. Pat. Nos. 5,435,445 and 5,465,822, on the other hand, disclose apparatus for sorting a variety of different articles by various methods, including analysis of the intensity of light reflected from an object through a polarizing filter. However, the use of light intensity analysis in the systems described in these patents is reserved for discriminating PET from vinyl, discrimination of other types of objects being carried out by color and self-induction analysis.
Another system for differentiating different types of plastic articles, as opposed to paper and plastic articles, using light intensity analysis, is disclosed in U.S. Pat. No. 5,518,124. The system described in this patent appears to be suitable for use with a single wavelength of electromagnetic radiation, but requires very high frequency radiation such as x-rays, microwaves, or gamma rays. X-ray and other high frequency radiation sources are not only more expensive than lasers in the visible or near visible wavelengths, but are also significantly more hazardous and therefore require expensive safety measures. In addition, at the wavelengths described in this patent, the problem of irregular object shapes becomes significant and must be taken into account. In fact, this patent explicitly states in col. 7 lines 53-55 that a "problem arises if only a threshold comparator (such as disclosed in Giovanni, [which involves a radiation chamber into which items are individually fed and removed] is used in an attempt to distinguish between the polyester and PVC containers," namely the thickness of the containers. According to this patent, thresholding cannot be used to distinguish the types of articles with which the patent is concerned.
U.S. Pat. No. 5,405,014 discloses analysis of the intensities of light passing through beverage bottles to determine the type of bottle and identify any residues in the bottles. In order to perform such a broad spectrum analysis, however, it is again necessary to use polychromatic light, and in particular to determine the attenuation of a broad spectrum of wavelengths. The use of a spectrometer, as opposed to a simpler monochromatic light detector, greatly increase the cost and complexity of the system.
U.S. Pat. No. 3,747,755 discloses a device that is capable of separating paper from plastic but requires analysis of specular reflections from objects passing the detector, and in particular attenuation of different infrared wavelengths. In the 26 years since issuance of this patent, the system does not seen to have been put into practice, at least with respect to separation of paper from plastic, although the principle of utilizing infrared reflectivity is also mentioned in a later patent, U.S. Pat. No. 5,590,791, in combination with color sensing.
U.S. Pat. No. 3,980,180 discloses a transmissive article sorting device which involves separation of glass particles by transmissivity, and in particular colored particles. The apparatus disclosed in this patent is similar to that utilized by the present invention, but the patent does not anywhere suggest a potential application to separation of paper and plastic articles which, unlike the differently colored glasses or ceramics in the patent, do not necessarily have transmission properties suitable for use in distinguishing the objects, particularly when utilizing the light source described in the patent, which is an LED rather than a laser. The use of an LED light source limits the apparatus described in the patent to separation of objects that are transparent or translucent in ordinary visible light.
Other references that might be of interest from the standpoint of background are U.S. Pat. Nos. 3,781,531 and 3,216,568, directed respectively to detection of flaws in a material by analysis of reflected laser light, and sorting of objects by detecting a chemiluminescence or phosphorescence effect.
The prior art thus includes systems that use polarization of polychromatic light to separate different types of plastics with otherwise similar transmissivities, attenuation spectra for wide ranges of different materials to be separated, high frequency x-rays or gamma ray sources, and infrared reflectivity. The one patent disclosing separation of paper uses infrared reflectivity, while another patent that discloses analysis of radiation absorption by different objects specifically teaches away from thresholding, at least when applied to different types of plastic.
The present invention, in contrast, has the following unique combination of features:
a monochromatic light source and single detector;
thresholding;
application to sorting of paper and plastic articles or objects.